A disc drive is a data storage device used to store digital data. A typical disc drive includes a number of rotatable magnetic recording discs that are axially aligned and mounted to a spindle motor for rotation at a high constant velocity. A corresponding array of read/write heads access tracks defined on the respective disc surfaces to write data to and read data from the discs.
Disc drive spindle motors are typically provided with a three-phase, direct current (dc) brushless motor configuration. The phase windings are arranged about a stationary stator on a number of radially distributed poles. A rotatable spindle motor hub is provided with a number of circumferentially extending permanent magnets in close proximity to the poles. Application of current to the windings induces electromagnetic fields that interact with the magnetic fields of the magnets to apply torque to the spindle motor hub and induce rotation of the discs.
Early disc drive spindle motor designs used Hall effect or similar external sensors to provide an independent indication of motor positional orientation. However, present designs avoid such external sensors and instead use electronic commutation and back electromagnetic force (bemf) detection circuitry to provide closed-loop spindle motor control. Such approach generally entails applying a predetermined sequence of commutation steps to the phase windings of the spindle motor over each electrical revolution (period) of the motor. A commutation step involves supplying the motor with current to one phase, sinking current from another phase, and holding a third phase at a high impedance in an unenergized state.
Detection circuitry measures the bemf generated on the unenergized phase, compares this voltage to the voltage at a center tap of the windings, and outputs a signal at a zero crossing of the voltages; that is, when the bemf voltage changes polarity with respect to the voltage at the center tap. The point at which the zero crossing occurs is then used as a reference for the timing of the next commutation pulse, as well as a reference to indicate the position and relative speed of the motor.
When a commutation state is selected is sense mode, a desired peak motor current is commanded through the DAC input to the power amplifier. The abrupt shut off of the selected switching element can cause negative current to be sourced out of ground through the sense resistor, motor winding, and the power supply.
One source for this transient current flow is from the deactivated phase. For example, if during a selected commutation state current is sourced to phase A and sunk from phase B, abruptly shutting off the driver forces negative current to be sourced out of ground through the sense resistor, through the lower field effect transistor (FET) body diode driving coil A, through the motor, back through the body diode of the upper FET on phase B to the power supply.
Negative current through the sense resistor generates a negative voltage across the sense resistor. Negative voltage that exceeds more than a diode voltage drop can create a parasitic current that overheats and damages the device. Also, sinking current into the power supply can cause an unstable condition in the power supply.
Forcing negative current back into a power supply also forces the supply voltage to increase. The increase in voltage is dependent on the current, capacitance and duration. Since power supplies are designed for minimum capacitance to keep costs down, power supplies are especially vulnerable. Voltage generated by a transient condition can easily exceed the limits in which the voltage supply breaks down.
Accordingly, there is a need for improvements in the art whereby a high performance spindle motor can be reliably controlled up to an operational velocity. It is to such improvements that the present invention is directed.